(1) Field of the Invention
The present invention relates to a system for monitoring the status of a plurality of repeaters located along transmission lines connecting terminal stations in a communication system.
(2) Description of the Related Art
Generally, in a communication system, wherein terminal stations are connected by transmission lines for data transmission, repeaters are located along the data transmission lines, and data is transmitted through the transmission lines by using the repeaters.
Each repeater in the communication system as mentioned above, usually is provided with the function of detecting and indicating the status of the operation and an alarm status of the repeater itself, e.g., an alarm status showing a power failure, a removable unit has been removed, or a break in an input line (hereinafter all of the above statuses are called as the repeater status).
Further, the current status of the repeaters located along a transmission line must be known to the terminal stations carrying out data transmissions through the transmission line by using the repeaters, i.e., the terminal stations must monitor the status of each repeater.
FIG. 1 shows a typical arrangement of a two-way digital data transmission system constructed between two terminal stations.
In FIG. 1, reference numerals 51 and 55 denote two terminal stations connected by transmission lines 56, 57, whereby a data transmission can be made in both directions. Reference numerals 52, 53, and 54 each respectively denote a two-way repeater, located along the transmission lines 56 and 57.
In a conventional method for monitoring the status of a plurality of repeaters located along the transmission lines connecting two terminal stations, one or each of the terminal stations 51 and 55 sends a polling signal (status request signal) to determine the status of each of the repeaters, receives an answer to the polling signal from each repeater individually, and thus collect the current status information from all of the repeaters.
The timing of the monitoring operation when the above mentioned conventional method for monitoring the status of repeaters is applied to the construction of FIG. 1, is shown in FIG. 2, and can be compared with the timing of the method of the present invention, which will be explained later.
In FIG. 2, the horizontal plane corresponds to a geometrical extension from the terminal station 51 to the terminal station 55 along the transmission lines 56 and 57 in FIG. 1, and the vertical plane corresponds to a direction of the progress of time. The arrows a, a', b, b', c, and c' indicated by a thinner line each show a signal (or data) transmitted by the above mentioned conventional method for monitoring a status of repeaters, where a, b, and c correspond to the aforementioned polling signals (status request signals) sent to the repeaters 52, 53, and 54, and a', b', and c' correspond to the aforementioned answers to the polling signals from the repeaters 52, 53, and 54, respectively.
The timing of the monitoring operation when the above mentioned conventional method for monitoring the status of repeaters is applied to the construction of FIG. 1, is shown in FIGS. 2 and 3. FIG. 2 shows the timing when a break has not occurred in the transmission lines, and FIG. 3 shows the timing when a break has occurred in a line in the construction of FIG. 1, for example, between the repeaters 52 and 53 on the transmission line 56. In FIGS. 2 and 3, the timing of the monitoring operation by the present invention is also shown and is explained later as a comparison with the conventional method described herein.
When a break does not occur in the transmission lines in the construction of FIG. 1, the operation of polling and receiving the answer to the polling by the terminal stations 51 is repeated for each of the repeaters 52, 53, and 54, and thus the terminal station can obtain the current status of all of the repeaters in the time "X", as shown in FIG. 2.
In the above mentioned conventional method for monitoring a status of repeaters, however, it takes a long time to collect the current status information from a plurality of repeaters located along a transmission line because the polling operation must be carried out at all of the repeaters on a one by one basis, as shown in FIG. 2.
In the above mentioned conventional method for monitoring a status of repeaters, if a break in a transmission line occurs in the construction of FIG. 1, for example, between the repeaters 52 and 53 on the transmission line 56, as shown in FIG. 3, the terminal station 51 receives the answer a' from the repeater 52 corresponding to the polling signal a from the terminal station 51, but does not receive the answers b' or c' from the repeaters 53 and 54, due to the line break. Therefore, throughout these operations, the terminal station 51 judges that a malfunction has occurred between the repeaters 52 and 53.
However, in the above conventional method, when a break has occurred in a transmission line, great inconvenience is caused because of the long time needed to determine whether or not a break has occurred between the terminal stations, and where the break has occurred.